Four-stroke engine

Abstract: An engine charge control system operates by initially reducing power from full load by retarding exhaust valve (20) closing timing to increase exhaust gas charge dilution up to the limit for efficient operation. Further load reductions are made by reducing charge mass without increased dilution via alternative steps such as throttling (in conjunction with intake check valves or using individual cylinder valves) or retarding intake valve (19) opening timing without throttling. Partial substitution of external charge dilution is also contemplated.

Description: This invention relates to external ignition or spark ignition internal combustion four-stroke engines. It is known in the art relating to engines to control the cylinder intake charges by various means and methods. These include intake manifold or individual cylinder throttling, variable intake and/or exhaust valve timing, auxiliary non-return or check valves (preferably reed valves) in the cylinder intake runners. US-A-5161497, which can be considered as the closest prior art, discloses a method of operating an engine by independently phase shifting the intake and exhaust valves, including delayed opening of the intake valve and delayed closing of the exhaust valve. The present invention seeks to provide an improved four-stroke engine. According to an aspect of the present invention, there is provided a method of operating a four-stroke engine as specified in claim 1. According to another aspect of the present invention, there is provided a method of operating a four-stroke engine as specified in claim 3. According to another aspect of the present invention, there is provided a four-stroke engine as specified in claim 13. Preferred embodiments of the invention may combine certain of the previously known means and can provide methods for their operation in the control of engine speed and load which can improve engine operating efficiency and/or emission control over substantial portions of the load range. A preferred embodiment combines variable timing of cylinder exhaust valve closing with manifold intake throttling and auxiliary cylinder intake check valves with optional bypass and an operating method which can maintain efficient operating conditions over the load range. A preferred operating method includes full load operation with fully open throttle and full power valve timing; reduced load operation in a high load range by delaying of exhaust valve closing to a first limit within a favourable combustion zone; optional further reduced load operation in an intermediate load range by throttling to reduce manifold pressure while substantially maintaining delayed exhaust valve closing at the first limit; and lower reduced load operation in a low load range obtained by further throttling to reduce manifold pressure while advancing exhaust valve closing from the first limit to near the initial full power valve timing at engine idle. An embodiment of the present invention is described below, by way of example only, with reference to the accompanying drawings, in which: Figure 1 is a schematic view of an engine with a simplified embodiment of charge control system; Figure 2 is a perspective view of portions of an engine incorporating the system of Figure 1; Figure 3 is a graph of engine load against manifold air pressure for a method of operating the engine of Figure 1; Figure 4 is a schematic view showing an engine with an alternative embodiment of charge control system; Figure 5 is a graph showing an alternative embodiment of method of operating the engine of Figure 4; Figure 6 is a graph comparing efficiency of the methods of Figures 3 and 5 with that of a baseline engine; Figure 7 is a schematic view of an engine with another alternative embodiment of charge control system; Figure 8 is a graph showing an alternative embodiment of method of operating the engine of Figure 7; Figure 9 is a schematic view of an engine with a third alternative embodiment of charge control system; and Figures 10A and 10B are graphs showing an alternative embodiment of method of operating the engine of Figure 9 and an expected efficiency improvement. Referring to Figure 1, a spark ignited four-stroke internal combustion engine 10 includes a cylinder block 11 having several cylinders 12, only one being shown. Each cylinder 12 has a piston 14 reciprocable therein and connected by a connecting rod 15 to a crankshaft 16. A cylinder head 18 closes the end of the cylinder 12 above the piston 14 and includes at least one intake valve 19 and one exhaust valve 20 controlling respectively an intake port 22 and an exhaust port 23. Variable timing valve actuation means 24 of any suitable type are provided. The intake port 22 connects with a manifold conduit 26 to form an intake passage 27 into which fuel is sprayed by a fuel injector 28. Upstream, an intake plenum 30 connects with the passage 27 of each engine cylinder and with an air intake tube 31 having a main throttle 32 of the butterfly type. An auxiliary one-way or check valve such as a reed valve 34 is located in each intake passage 27 between the plenum 30 and the fuel injector 28. The presence of the reed valves 34 prevents or limits backflow of gases from the engine cylinders 12 into the intake manifold plenum 30 and thus permits use of camshaft phase control timing of all the valves to obtain the desired retarding of the closing of the exhaust valve. In an embodiment preferred for its simplicity and illustrated in Figure 2, the variable timing valve actuation means 24 comprises a cam phaser 35 capable of varying the phase angle of the camshaft 36 (which actuates both the intake and exhaust valves 19, 20 through valve gear not shown) relative to the phase angle of the crankshaft 16 by which the camshaft is driven. The phaser 35 is driven by the crankshaft through a chain 38 and is in turn connected through a gear train 39 to drive a balance shaft 40 at crankshaft speed with a fixed phase angle. An internal planetary gear mechanism in the phaser is adjustable through a control shaft 42 to vary the camshaft phase angle. Details of an engine application including a phaser of this type can be found in US-A-5,327,859. A preferred method of operating the engine 10 of Figures 1 and 2 is illustrated in Figure 3 which shows computer simulated operational data where the engine load, represented by net mean effective cylinder pressure (NMEP), is plotted against manifold air pressure (MAP), which indicates relative throttle position. The fine lines 43, 44, 45, 46, 47 indicate operation over the range of throttle settings and, respectively, at a baseline 43 of 0° without check valves, and then with check valves at 0°, 10°, 30° and 50° retard of the exhaust valve closing timing which, in this case, represents retarding of the timing of all the valves, intake and exhaust. The dashed portions of the lines indicate zones in which inefficient combustion would result. The short sloping segment 48 of the bold line represents operation with reducing throttle opening at 70° of valve timing retard, which is chosen as the operating limit in this case to ensure that the intake valves are closed when ignition of the cylinder charges occurs. The horizontal upper segment 50 of the bold line represents operation at constant throttle position (constant MAP) with valve timing (exhaust valve closing) varying between 0° and 70° retard. The sloping lower segment 51 of the bold line represents operation with reducing throttle opening while the valve timing is being advanced back towards the full load setting. The bold line 50, 48, 51 of Figure 3 represents the preferred conditions for operating the engine at a selected speed over its complete load range. A different set of conditions will exist for each operating speed or increment within the engine operating speed range. Reduction of power in Figure 3 from full load to near idle is as follows. Point 52 indicates engine operation at full load or power output (1000 kPa NMEP at 100 kPa MAP). Here, the main throttle is held fully open and the camshaft timing is set to obtain maximum power. The cams provide considerable overlap of the valve open periods for each cylinder to maximise the intake charge at peak output speed. To reduce engine load down to about 460 kPa NMEP, the exhaust valve closing timing is retarded progressively along segment 50 up to 70° to point 54 while throttle position is held constant. Since 70° is the maximum retard allowed for this embodiment, further power reduction down to about 400 kPa NMEP is obtained by moving along segment 48 to point 55 by partially closing the throttle (reducing MAP) while holding valve timing retard constant at 70°. Further power reduction is along segment 51 to point 56 and combines gradual closing of the throttle with corresponding advance of the valve timing from the retarded condition back to or near the initial operating condition, the operating conditions along the segment 51 being controlled to maintain combustion conditions at their most efficient points within the efficient operating zone bordered by segment 51. Figures 4 and 5 illustrate an alternative embodiment of engine and method of controlling the engine. Engine 58 is similar to engine 10 of Figure 1 except that the variable timing valve actuation means 59 acts only upon the exhaust valves 20 of the engine. Any suitable variable timing device may be used which is capable of retarding the exhaust valve closing event a substantial amount of up to about 100° of crankshaft rotation from the nominal full load position. While a continuous variation over the range is preferable, a multiple-step or two-step valve lifter and cam arrangement similar to that disclosed in US-A-5,090,364 might be used. Figure 5 illustrates a computer simulation of the operation of engine 58 having continuously variable exhaust valve closing timing. Fine lines 60, 61, 62, 63, 64, 66, 67 indicate operation at baseline of 0° without check valves and then with check valves with 0°, 10°, 30°, 50°, 70° and 90° of exhaust valve retard from the nominal full load phase setting. The bold line made up of segments 68, 70 comprises the operating line for most efficient operation. Starting at the full load point 71, load is reduced along segment 68 to point 72 solely by progressively delaying the timing of exhaust valve closing (EVC) up to about 100°. Further timing retard would cause engine operation, in the simulated instance, to fall outside of the zone of acceptable combustion performance (to the right of segment 70). Accordingly, additional load reduction is accomplished by moving down segment 70 by progressively closing the throttle 32 while returning EVC timing from the maximum at point 72 to near the initial 0° retard reached at point 74. Figure 6 shows the comparative thermal efficiencies computed for a baseline engine without intake reed valves or variable exhaust valve timing shown by line 75, an engine as in Figure 1 with reed valves and variable camshaft phasing (VCP) shown by line 76 and an engine as in Figure 4 with reed valves and variable exhaust valve closing (VEC) shown by line 78. The greater high load efficiency of the VEC timing (line 78) is attributed in part to the limitation on timing retard in the VCP system (line 76) which varies the timing of both intake and exhaust valves. If desired, further enhancement in engine performance of the two embodiments described having intake check valves 34 may be obtained by adding to their systems the features disclosed in either US-A-4,986,225 or US-A-4,991,547. Another alternative embodiment is illustrated in Figures 7 and 8. Figure 7 shows an engine 79 similar to engine 10 of Figure 1 except that the reed valve 34 and the main throttle 32 are replaced by individual cylinder throttles 80 located in the intake passages 27 between the intake plenum 30 and the fuel injectors 28. The preferred method of operation is indicated in Figure 8 at 1500 rpm as an example. Fine lines 82, 83, 84, 85, 86, 87 indicate retard of the valve timing, or particularly the timing of exhaust valve closing, of 0°, 10°, 30°, 50°, 70° and 90°, respectively. The bold line made up of horizontal segment 88 and downwardly curving segment 90 indicates the manner of engine operation. In operation, engine load or power control is accomplished by a combination of control of the timing of exhaust valve closing and throttling with the individual cylinder throttles in the intake passages 26. Full power operation occurs with the throttles 80 sufficiently open and the normal unretarded timing of exhaust valve closing. This occurs at or near point 91 of Figure 8. Load reduction to the lower limit of efficient combustion (about 600 kPa NMEP) at point 92 occurs along line 88 by progressively retarding exhaust valve closing from 0° to 90° while the throttles 80 remain fully open. Further load reduction along line 90 requires progressive closing of the throttles and advancing of the exhaust valve closing timing back to about its full load timing when the throttles are at minimum load position. The restriction of the individual cylinder throttles has an effect similar to that of the reed valves used in other embodiments in restricting the backflow of exhaust gases into the manifold. Restriction in the cylinder inflow direction also causes an effective intake delay similar to that obtained by retarding exhaust valve closing, as described below. Figures 9, 10A and 10B disclose another embodiment in which engine 94 in Figure 9 differs from the engine 58 of Figure 4 in the omission of the main throttle 32 and the reed check valve 34 of the prior embodiments. Instead engine 94 includes a variable timing valve actuator 95 for the intake valve(s) 19 in addition to the previous valve actuator 59 for the exhaust valve(s) 20. As before, the actuator 59 is capable of retarding the timing of exhaust valve closing up to 100° or more from the normal full load setting. Similarly, the added actuator 95 is capable of retarding the timing of the intake valve opening by a similar amount. A method for controlling the engine 94 is illustrated in Figures 10A and 10B at 1500 rpm. In Figure 10A, solid line 96 represents intake valve opening timing and dashed line 98 represents exhaust valve closing timing, both in crank angle degrees at an indicated percent of full load. In Figure 10B, solid line 99 indicates the predicted percent improvement in thermal efficiency at the selected load values. At full (100%) load, the valve timing is set for maximum power at the points indicated for 100% load. To reduce power, the initial step is to retard exhaust valve closing (EVC) timing without changing the other timing parameters. As shown, a retard from about 15° to about 55° ATDC (after top dead centre) lowers the load to about 90% by introducing exhaust gas charge dilution. Further load reduction to about 55% is accomplished by further retarding EVC to about 110° ATDC while also retarding intake valve opening (IVO) timing a similar amount from about 15° BTDC (before top dead centre) to about 40° ATDC. This limits the admission of fresh charge and replaces it with additional exhaust gas up to the limit for efficient cylinder combustion. Thereafter, further load reduction requires limiting the mass of the charge without further dilution. This could be accomplished by throttling as in the earlier disclosed embodiments. However, in this instance, the IVO is further retarded to reduce the fresh charge admitted and the EVC is similarly advanced to reduce the admission of exhaust gas in a proportional manner. If complete load reduction to idle operation cannot be obtained by this method, then intake throttling can be introduced as necessary to reach the desired lower load conditions. As indicated by line 99 in Figure 10B, the method is projected to obtain efficiency over a conventionally throttled engine increasing up to about 10% at maximum dilution, occurring about 55% load, and holding this improvement through an indefinite range of lower partial load points in which throttling is not required. It will be apparent that the various methods and embodiments described above as examples have in common the following elements of their methods: 1. Initially, engine load or power is reduced by retarding EVC timing to increase charge dilution with exhaust gas up to an operating limit while reducing engine pumping work loss without reducing the effective cylinder compression ratio; 2. Secondarily, the mass of the cylinder charge is further reduced by throttling the inlet or retarding IVO timing to obtain a similar effect while preventing further dilution by advancing exhaust valve closing timing. The reed check valves used in the first two embodiments prevent exhaust backflow and also provide a resistance which favours the flow of exhaust gas over fresh charge into the cylinder as long as the exhaust valve is open. The cylinder throttles of the third embodiment have a similar effect. In the last embodiment described, retarding of IVO timing provides this function. The bypass flow and external exhaust recirculation systems of US-A-4,986,225 and US-A-4,991,547 as well as other external exhaust recirculation means may be substituted for some of the internal charge dilution measures described if desired. Each cylinder may have at least two exhaust valves, the closing time of one of which remaining constant during engine operation.

Claims: A method of operating a four-stroke engine, which engine includes at least one cylinder (12) comprising at least one intake valve (19), at least one exhaust valve (20), a piston (14) in the cylinder connected to a crankshaft (16) and timing means (24) for varying intake charge pressure provided to the intake valves and means for varying the timing of exhaust valve closing relative to the crank angle of the associated piston; the method comprising the steps of providing a desired maximum intake charge pressure to the intake valve and a selected valve timing for operation of the engine at substantially maximum rated power at various operating speeds; reducing power for operation in a high power range immediately below maximum power by increasingly delaying exhaust valve closing combined at least in part with proportionally delaying intake valve opening up to a maximum delay selected to avoid adversely affecting charge combustion in the cylinders; and completing power reduction for operation in a low power range above idle by further delaying intake valve opening and advancing exhaust valve closing timing back to substantially the initial timing setting. A method according to claim 1, wherein an initial portion of the reduction from maximum power is accomplished while maintaining intake valve opening timing substantially constant. A method of operating a four-stroke engine, which engine includes at least one cylinder (12) comprising at least one intake valve (19) and at least one exhaust valve, a piston (14) in the cylinder connected to a crankshaft (16), a throttle (80) for varying intake charge pressure provided to the intake valve and timing means (24) for varying the timing of exhaust valve closing relative to the crank angle of the associated piston; the method comprising the steps of providing a desired maximum intake charge pressure to the intake valves and a selected valve timing for operation of the engine at substantially maximum rated power at various operating speeds; reducing power for operation in a high power range immediately below maximum power by increasingly delaying exhaust valve closing up to a maximum delay selected to avoid adversely affecting charge combustion in the cylinders while the throttle is held fully open; and completing power reduction for operation in a low power range above idle by reducing intake charge pressure to the intake valves by throttling while advancing exhaust valve closing timing back to substantially the initial timing setting. A method according to claim 3, including the step of reducing power from the high power range to an intermediate power range above the low power range by reducing intake charge pressure while maintaining exhaust valve closing delay substantially constant. A method according to claim 3 or 4, wherein the or each cylinder includes at least two exhaust valves, the exhaust valve closing timing of at least one of the valves remaining substantially constant during engine operation. A method according to claim 3, 4 or 5, wherein control of the intake charge pressure is provided by a manifold intake throttle (32). A method according to claim 3, 4 or 5, wherein control of the intake charge pressure is provided by a cylinder intake conduit throttle (80). A method according to any preceding claim, wherein control of the exhaust valve closing timing is provided by a variable valve timing mechanism (59). A method according to any preceding claim, wherein control of the exhaust valve closing timing is provided by camshaft phase changing means which alters the timing of a camshaft (42) that actuates the valve or valves of the or each cylinder. A method according to claim 9, wherein the camshaft also actuates the intake valve or valves of the or each cylinder. A method according to claim 3, wherein intake charge pressure is determined in part from a cylinder intake check valve (34) upstream of the or each intake valve. A method according to claim 11, wherein control of the intake charge pressure is provided by a manifold intake throttle (32) together with the cylinder intake check valve. A four-stroke engine including at least one cylinder (12) comprising at least one intake valve (19) and at least one exhaust valve (20), a piston (14) connected to a crankshaft (16), a throttle (80) for varying intake charge pressure provided to the intake valve, timing means (24) for varying the timing of exhaust valve closing relative to the crank angle of the associated piston, characterized by control means for operating the engine to provide a desired maximum intake charge pressure to the intake valves and a selected valve timing for operation of the engine at maximum rated power at various operating speeds; to reduce power for operation in a high power range immediately below maximum power by increasingly delaying exhaust valve closing up to a maximum delay selected to avoid adversely affecting charge combustion in the cylinders while the throttle is held fully open; and to complete power reduction for operation in a low power range above idle by reducing intake charge pressure to the intake valves by throttling and advancing exhaust valve closing timing back to substantially the initial timing setting. A four-stroke engine according to claim 13, wherein the control means is operative to reduce power from the high power range in an intermediate power range above the low power range by reducing intake charge pressure while maintaining exhaust valve closing delay substantially constant. A four-stroke engine according to claim 13 or 14, wherein the or each cylinder (12) includes at least two exhaust valves, the control means being operative to keep the exhaust valve closing timing of at least one of the valves substantially constant during engine operation. A four-stroke engine according to claim 13, 14 or 15, including a manifold intake throttle (32) actuatable by the control means to provide control of the intake charge pressure. A four-stroke engine according to claim 13, 14 or 15, including a cylinder intake conduit throttle (80) for the or each cylinder actuatable by the control means to provide control of the intake charge pressure. A four-stroke engine according to any one of claims 13 to 17, including camshaft phase changing means operable to vary the timing of a camshaft (42) which actuates the exhaust valve or valves of the or each cylinder, the phase changing means being actuatable by the control means to provide control of the exhaust valve closing timing. A four-stroke engine according to claim 18, wherein the camshaft also actuates the intake valve or valves of the or each cylinder. A four-stroke engine according to any one of claims 13 to 19, wherein the pressure varying means includes a cylinder intake check valve (34) upstream of the intake valve or valves. A four-stroke engine according to claim 20, wherein the chek valve is a reed valve.

Inventors: WU KO-JEN, WU, KO-JEN
Applicants: GEN MOTORS CORP, GENERAL MOTORS CORPORATION
Application year: 1994
Main classification: F02D13
IPC codes: F02B75, F01L1, F02M25, F02D9, F02B29, F02D13, F02D43